Improve DynamicDeps docs.

docs/dynamic-deps.rst

@@ -46,9 +46,15 @@ request meta dictionary using the "inject" key:
 
 
         def parse_book(self, response, book_page: BookPage, dynamic: DynamicDeps):
+            # access the dynamic dependency values by their type:
             other_dep = dynamic[OtherDep]
             ...
+            # or get them and their types at the run time:
+            for dep_type, dep in dynamic.items():
+                if dep_type is OtherDep:
+                    ...
 
 The types passed this way are used in the dependency resolution as usual, with
 the created instances available in the :class:`scrapy_poet.DynamicDeps
-<scrapy_poet.injection.DynamicDeps>` instance.
+<scrapy_poet.injection.DynamicDeps>` instance, which is a dictionary with
+dependency types as keys and their instances as values.